Seismic exploration of subsurface geophysical structures on land and offshore is a widely used technique for searching for oil or gas. Specifically, reflection seismology is a method of geophysical exploration used to image the subsurface structure in order to evaluate whether oil and/or gas reservoirs may be present. There is continual interest in obtaining better images of the subsurface structure, based on shorter surveying periods.
In reflection seismology, seismic waves are artificially generated and directed toward the explored subsurface structure. In a marine setting, reflected compressional waves recorded by hydrophones and/or accelerometers are widely used. In other settings (e.g., land and ocean-bottom surveys), information on reflected shear waves may also be acquired. Analysis of the arrival times and amplitudes of these reflected waves is the basis for generating an image of geological layers.
FIG. 1 is a bird's-eye view of a data acquisition system 10 used in marine seismic explorations. The term “marine” is not limited to sea or ocean environments, but such systems may be used in any large bodies of water (e.g., freshwater lakes). The data acquisition system 10 includes a ship 2 towing plural streamers 4 (also known as spreads) that may extend over kilometers behind ship 2. Seismic detectors 6 (only a few are labeled) are disposed along streamers 4. Each streamer 4 has attached positioning devices (not shown) such as birds, floaters, deflectors, etc., configured and operated to maintain the towed streamers' geometry (i.e., each streamer's depth profile and all streamers parallel to one another). Ship 2 may also tow one or more seismic sources 8 (which may include plural source arrays) configured to generate seismic waves. A distance between source 8 and the first seismic detectors on streamers 4 may be a few hundred meters, while streamer length may be up to 10 kilometers. The seismic waves generated by source 8 propagate downward to partially reflect off of, and penetrate, the seafloor. Seismic waves penetrating the seafloor may then be reflected by one or more reflecting structures, such as layer interfaces (not shown in FIG. 1) inside the explored underground structure. The reflected seismic waves travel upward and may be detected by seismic detectors 6. When the same ship tows the seismic source and the streamers along a sail-line S, the acquired data has a narrow, limited azimuth angle range. At the front of the spread, the azimuth can be 75° but rapidly decreases to less than 10°. Azimuth (e.g., α in FIG. 1) is defined in a horizontal plane relative to a towing direction such that if a seismic detector is positioned behind the source in a first towing direction the azimuth angle is 0°. Note that if data is acquired while covering the same surface area while towing the system back and forth, the azimuth definition is maintained. Therefore, when the system is towed in a second direction opposite to the first direction, if a seismic detector is positioned behind the source, the azimuth angle is 180°. Thus, data acquired while the system is towed in the first direction corresponds to an azimuth range of 0°±75° and data acquired covering while the system is towed in the second direction corresponds to an azimuth range of 180°±75°.
To achieve higher-resolution images of the subsurface, a wide azimuth (WAZ) data acquisition technique has been developed in the past years. Using this technique, one or more seismic sources are towed laterally relative to the ship towing the streamer set carrying the detectors.
Use of conventional WAZ data acquisition systems has proven to be challenging because weather time windows in which to perform seismic surveys may be scarce and short. If the target area is large, a WAZ survey may not be completed in one season and may require survey campaigns spanning multiple years. Given the large amount of equipment and large number of personnel involved, WAZ surveys are also expensive. The costs and uncertainty of completing a WAZ acquisition may then render WAZ seismic surveys unattractive, if at all feasible.
Therefore, it is desirable to provide WAZ methods and seismic data acquisition systems that shorten WAZ survey duration without compromising, and potentially improving, seismic data quality.